1. Field of the Invention
The present invention relates to LED drive circuits for driving an LED (light-emitting diode), and to such LED lamps, LED lighting appliances, and LED lighting systems as use an LED as light sources.
2. Description of Related Art
For their advantages such as low current consumption and long lifetimes, LEDs have been widening their application beyond display apparatuses into lighting equipment and the like. In many pieces of LED lighting equipment, a plurality of LEDs are used to obtain the desired luminance.
Many common pieces of lighting equipment use a commercial AC (alternating-current) 100 V power source, and when consideration is given to cases such as where LED lamps are used in place of common lamps such as incandescent lamps, it is desirable that LED lamps are configured to use, like common lamps, a commercial AC 100 V power source.
For the control of the light of incandescent lamps, phase-control light controllers (generally called incandescent light controllers) are used, which allow easy control of light by means of a variable-resistance device though the control of the power supplied to the incandescent lamp achieved by turning a switching device (commonly, a thyristor device or a triac device) on at a given phase angle of an AC power source voltage. An example of the configuration of an incandescent lamp lighting system provided with an incandescent lamp and a phase-control light controller is shown in FIG. 14.
The incandescent lamp lighting system shown in FIG. 14 is provided with a phase-control light controller 2 and an incandescent lamp 9. The phase-control light controller 2 is connected in series between an AC power source 1 and the incandescent lamp 9. In the phase-control light controller 2, when the knob (unillustrated) of a potentiometer Rvar1 is set at a given position, a triac Tri1 is turned on at the power phase angle corresponding to that position. In the phase-control light controller 2, a noise prevention circuit is also provided which is constituted by a capacitor C1 and an inductor L1, and the noise prevention circuit reduces the terminal noise that returns from the phase-control light controller 2 to the power line.
An example of the voltage and current waveforms at relevant points in the incandescent lamp lighting system shown in FIG. 14 is shown in FIG. 15A, and an enlarged diagram of the period P in FIG. 15A is shown in FIG. 15B. In FIGS. 15A and 15B, VOUT2, I2, and I9 respectively indicate the waveform of the output voltage of the phase-control light controller 2, the waveform of the current passing through the triac Tri1 in the phase-control light controller 2, and the waveform of the current passing through the incandescent lamp 9. In the example shown in FIGS. 15A and 15B, immediately after the triac Tri1 is triggered and turned on, the current passing through the triac Tri1 undulates (swings up and down) several times; when it swings down for the first time, the current passing through the triac Tri1 becomes negative and lower than the holding current, and this means that the triac Tri1 is momentarily turned off immediately after being turned on. Nevertheless, flickering is small, and light control can be done as normally.
It is however known that, with incandescent lamps of lower wattages, flickering and blinking prevent normal light control.
For the control of the light of LED lamps that use an AC power source, it is desirable, as for the control of the light of incandescent lamps, to use phase-control light controller. Now, a conventional example of an LED lighting system that can control the light of LED lamps that use an AC power source is shown in FIG. 16. In FIG. 16, such parts as find their counterparts in FIG. 14 are identified by common reference signs, and no detailed description of such parts will be repeated.
The LED lighting system shown in FIG. 16 is provided with a phase-control light controller 2, an LED drive circuit including a diode bridge DB1 and a current limit circuit 5, and an LED module 3. The phase-control light controller 2 is connected in series between an AC power source 1 and the LED drive circuit.
An example of the voltage and current waveforms at relevant points in the LED lighting system shown in FIG. 16 is shown in FIG. 17A, and an enlarged diagram of the period P in FIG. 17A is shown in FIG. 17B. In FIGS. 17A and 17B, VOUT2, I2, and I3 respectively indicate the waveform of the output voltage to the phase-control light controller 2, the waveform of the current passing through a triac Tri1 in the phase-control light controller 2, and the waveform of the current passing through the LED module 3. In the example shown in FIGS. 17A and 17B, immediately after the triac Tri1 is triggered and turned on, the current passing through the triac Tri1 undulates (swings up and down) several times; thus, when the triac Tri1 is turned on at a given phase angle, a wavefoini as if oscillating results, and light control cannot be done normally. As shown in FIG. 17B, which is an enlarged view of the period P in FIG. 17A, after the current passing through the triac Tri1 undulates in the positive and negative directions several times, the triac Tri1 is turned off, and thereafter it is triggered again; thus, the current passing through the triac Tri1 undulates in the positive and negative directions several times, and then the triac Tri1 is turned off—so repeats the same sequence of events. How this occurs is as follows: when the current passing through the triac Tri1 turns from positive to negative, it becomes equal to or lower than the holding current; after the triac Tri1 is turned off, it does not respond for a certain period, and even after this period has elapsed, the current passing through the triac Tri1 remains lower than the holding current until it is triggered next time.
Due to differences in lighting characteristics between incandescent lamps and LEDs, failure of normal light control as described above is more likely to occur in LED lighting systems than in incandescent lamp lighting systems.
JP-A-2006-319172 discloses an LED lighting system as shown in FIG. 18. The LED lighting system shown in FIG. 18 is provided with a phase-control light controller 2, a diode bridge DB1, a current holding means, a rectifying-and-smoothing means, and a LED module 3. The phase-control light controller 2 is connected in series between an AC power source 1 and the diode bridge DB1, and the current holding means and the rectifying-and-smoothing means are provided between the diode bridge DB1 and the LED module 3.
The current holding means is composed of resistors R181 to R186, Zener diodes ZD1 and ZD2, transistors Q181 and Q182, and a capacitor C181. In the current holding means, when the source voltage outputted from the AC power source 1 is equal to or lower than 100 V, the transistor Q182 is on, and passes a current corresponding to the holding current of a triac Tri1 in the phase-control light controller 2; when the source voltage is not equal to or lower than 100 V, the transistor Q182 is off. The transistor Q182 passes a current (about 30 mA) such that the current through the triac Tri1 in the phase-control light controller 2 is not equal to or lower than the holding current.
In the current holding means described above, however, the period during which the corrector current of the transistor Q182 passes is the period after the transistor Q182 turns on until the transistor Q181 turns on, and the transistor Q181 turns on when, after the triac Tri1 in the phase-control light controller 2 turns on, the Zener diode ZD1 turns on. Thus, for example, if the triac Tri1 in the phase-control light controller 2 conducts abruptly, or if the source voltage of the AC power source 1 becomes high, the on-state period of the transistor Q182 may be so short and accordingly the period during which the current that does not become equal to or lower than the holding current of the triac Tri1 passes may be so short that the triac Tri1 cannot turn on.